This invention relates generally to marine anchors, and more particularly to a drag embedment anchor.
Drag embedment anchors are comprised of two major components, a fluke and a shank. Generally, the fluke is relatively flat and has a large surface area, with two pointed front tips which penetrate the soil on the sea floor as the anchor is dragged. When the anchor is completely embedded in the soil on the sea floor, the pressure of the soil on the fluke is a major component of the holding power of the anchor. A typical fluke is formed from a flat plate stiffened by external ribs, or from a wedge-shaped box stiffened by internal ribs.
The shank is generally a long, thin member which is fastened near the stern of the fluke at one end, and to a mooring line at the other end. In most anchors, the shank is coincident with the central longitudinal axis of the fluke when the anchor is viewed from above. The shank serves to transmit forces between the fluke and the mooring line.
There are two broad categories of drag embedment anchors within which most anchors can be classified. The first category includes traditional or swing shank anchors. Traditional anchors have shanks which are straight and rotatably secured to the fluke at a single hinge point so that the shank can pivot to a limited degree on either side of the fluke. As the anchor is dragged along the sea floor, one side of the fluke will face downward, toward the sea floor. Once the fluke tips penetrate the soil, the shank will swing to the other side of the fluke. Since either side of the fluke can be facing downward, the fluke is symmetrical in shape.
The second broad category of anchors includes the modern, or fixed shank type. The flukes of these anchors have a defined top surface and underside since the attitude of the shank is fixed relative to the fluke during operation. The shank extends upwardly from the top surface of the fluke. In order to be able to penetrate the soil, most of these anchors must land on the sea floor with the fluke beneath the shank and with the fluke's bottom side resting on the sea floor.
An important parameter for measuring anchor performance is the holding efficiency, or the ratio of the holding power over the weight of the anchor. Due to the fluke symmetry of the swing shank anchors, extra weight is added to the anchor, thus reducing efficiency. Fixed shank anchors eliminate some of the redundant structure of the traditional anchors.
Anchor designs of both types may be adjusted to accommodate varying soil conditions. The attitude of the shank relative to the fluke should be changed to assist the initial penetration and ultimate depth of the flukes within the soil. The softer the soil is, the wider the "fluke opening angle" should be. However, the means for adjusting the fluke opening angle on previous anchors have suffered from various drawbacks. In swing shank anchors, this angle can be varied by fastening a stopper to the fluke which limits the rotation of the shank past a certain point. However, the stopper is removed when a wide fluke opening angle is desired, and must be stored and handled when not in use. Further, the weight of the stopper reduces efficiency of the anchor when the stopper is in use. Fixed shank anchors such as the "Stevshark", manufactured by Vryhof Ankers, Holland, have shanks which can be fixed at various attitudes without removable parts. However, to adjust the fluke opening angle, the Stevshark requires the laborious fastening and unfastening of a plurality of nuts and bolts which secure the shank to the fluke. Some of the fixed shank anchors do not have any angle adjustment option.
Another problem for both types of anchors is that of initially penetrating the soil as the anchor is being dragged along the sea floor. Previous swing shank anchors have utilized "tripping palms", formed by widening the stern of the fluke to create a V-shaped base on which the anchor rests when the fluke and shank are both in a vertically upright position. As the anchor is pulled by the mooring line, the tripping palms ensure that the fluke will "trip" over and engage the sea floor so that the fluke tips will penetrate the soil, and so the shank will swing open relative to the fluke. While tripping palms have proved successful, their added weight detracts from the anchor's holding efficiency, especially since after the anchor has fully penetrated the soil, the tripping palms serve no further purpose.
Another drawback of previous anchor designs has been their poor control of roll and yaw instability both before and after the anchor has fully penetrated the soil. Yaw is defined as rotation of the anchor about an axis which is normal to the top surface of the fluke, while roll is rotation about the central longitudinal axis of the fluke. Most previous anchors have had their shanks attached near the rear or stern of the fluke, which is far behind the fluke's pressure center. The pressure center is defined as the point on the top surface of the fluke through which the resultant force due to the soil pressure passes. Due to the relative location of the pressure center and the shank attachment point, the shank is effectively pushing the fluke forward, which creates instability.
When the fluke's tips encounter uneven loading, the anchor will yaw. The forces acting on the fluke tips which cause yaw will also cause rolling of the anchor. As the anchor yaws, the shank becomes angled relative to the mooring line. When the yawing force acting on the fluke tips is coupled with a component of the mooring line force on the shank, a roll moment is created.
To stabilize yaw and roll, traditional anchors have been designed with outriggers or stocks, which are elongated tubular bars that protrude outwardly to the sides of the fluke and generally normal to the axis of the shank. While stocks have been helpful in minimizing yaw and roll, their weight reduces the overall efficiency of the anchor.
Thus, there is a need for a drag embedment marine anchor which is lightweight, efficient, adjustable for various soil conditions, easily makes initial penetration, and stabilizes yaw and roll.